A hydraulic excavator, which is a typical example of a civil-engineering or construction machines, comprises a swing as one of a plurality of working elements. A hydraulic drive system for the swing generally comprises a hydraulic pump constituting a hydraulic-fluid source, a hydraulic motor for driving the swing, a directional control valve for controlling flow of hydraulic fluid supplied from the hydraulic pump to the hydraulic motor, a pair of main lines through which the directional control valve and the hydraulic motor are connected to each other, the pair of main lines serving selectively as a fluid supply line and a fluid return line by switching of the directional control valve, and a pair of relief valves provide respectively in lines through which the pair of main lines are connected to each other. Further, since the swing is an inertial body, it is necessary to brake the hydraulic motor upon halting movement of the swing and, accordingly, brake means utilizing back pressure of the hydraulic motor is incorporated in the hydraulic drive system. The brake means is a counter balance valve arranged in the pair of main lines, for example.
The counter balance valve operates such that the hydraulic fluid is prevented from being returned to a tank from the main lines when the directional control valve is returned to a neutral position in order to halt the swing from a condition under which the swing is driven. When the hydraulic motor tends to be rotated by an inertial force of the swing, the hydraulic fluid is prevented from being returned from the main line on a return side of the hydraulic motor by the counter balance valve. By doing so, a pressure (back pressure) in the main line increases abruptly by a pumping action of the hydraulic motor. When a magnitude of the pressure exceeds a set pressure of a corresponding one of the relief valves, the relief valve is moved to an open position. Thus, the hydraulic fluid is recirculated through a closed circuit composed of the relief valve, the hydraulic motor and the main lines, so that the hydraulic motor is braked.
By the way, in the hydraulic drive system provided with such brake means, there is such a problem that a reverse motion of the inertial body occurs due to the action of the back pressure upon halt of the inertial body.
That is, as described above, the pressure in the main line on the return side of the hydraulic motor increases abruptly to the set pressure of each of the relief valves to brake the hydraulic motor. When the hydraulic motor is halted, however, each of the relief valves is moved to the closed position so that the pressure in the main line is brought to a condition maintained at high pressure. Accordingly, a differential pressure occurs between outlet and inlet ports of the hydraulic motor. The hydraulic motor begins to rotated reversely by the differential pressure. Thus, the differential pressure between the outlet and inlet ports of the hydraulic motor is nullified. However, the hydraulic motor continues to be further rotated in the same direction by the inertial force of the swing. Thus, this time, the pressure in the main line on the reverse side is brought to a high pressure so that a differential pressure occurs across the hydraulic motor. The hydraulic motor again begins to rotate. In this manner, in the case where the brake means utilizing the back pressure is provided, a reverse motion occurs in which the swing is swung a plurality of times, by the inertial force of the swing, in spite of the fact that halt of the swing is intended.
In order to solve the above-described problem, JP,A, 57-25570 has proposed a pair of reverse-motion check valves each of which is connected to a pair of main lines of a hydraulic drive system. Each of the reverse-motion check valves is arranged as follows. That is, a volume chamber is defined between a valve housing and a movable seat against which a poppet is abutted. A small bore for damping is provided through which the hydraulic fluid is returned from the volume chamber. Return speed of the movable seat is slowed or retarded with respect to the poppet by a restricting action of the damping small bore immediately after halt of the hydraulic motor. The poppet and the movable seat are temporarily spaced from each other to move the valve to the open position. A high pressure generated in the main line on the return side of the hydraulic motor is relieved to the other main line. The pressure in the main line on the return side is reduced by the temporary movement of the reverse-motion check valve to the open position. Energy required for the reverse motion of the swing disappears before the reverse-motion check valve is again moved to the closed position. Thus, no reverse motion of the swing occurs.
As described above, each of the reverse-motion check valves disclosed in JP,A, 57-25570 is moved to the open position by utilization of the restricting action of the damping small bore. In the case of a low-temperature environment and in the case where the hydraulic excavator is arranged on a slope, however, there occurs a problem. That is, when the hydraulic fluid is low in temperature, the hydraulic fluid increases in viscosity. Accordingly, the restricting action of the damping small bore increases so that the return speed of the movable seat is retarded. Thus, a condition under which the reverse-motion check valve is opened continues long. Accordingly, in the case where any one of the pair of main lines is brought to a high pressure accompanied with halt operation of the swing, the reverse-motion check valve continues, for a relatively long time, the condition under which the reverse-motion check valve is opened as described above. Accordingly, the hydraulic fluid in one of the main lines flows into the opposite main line so that a swing motor is rotated abnormally contrary to intention. After all, there occurs such a situation that the swing is rotated transiently. Such transient rotation reduces working efficiency and, in addition thereto, reduces safety so that operability is considerably impeded.
It is an object of the invention to provide a hydraulic drive system for a civil-engineering and construction machine, capable of preventing a reverse motion of an inertial body upon halt of an actuator, and capable of ensuring that the actuator is halted without being accompanied with abnormal operation at low-temperature environment and when the civil-engineering and construction machine is arranged on a slope to conduct operation.